This invention relates to the collection for analysis or other purpose of chemical compounds extracted by supercritical fluid from samples of interest. Such extraction is known as supercritical fluid extraction (SFE).
Extraction of chemical compounds or elements from complex mixtures of chemical compounds or elements is important in many industries and disciplines. Complex extraction techniques and apparatuses have been developed to isolate compounds or elements of interest in pollution samples, soil samples, biological tissue, drugs, oils, metals, and thousands of other substances and matrices. The compounds or elements are extracted from the samples through various techniques, and once isolated, they are collected by some technique and used, further processed, or analyzed.
In SFE, the sample is exposed to a supercritical fluid solvent (typically CO.sub.2) under supercritical conditions. A supercritical fluid exists when a material is near or above its critical temperature and pressure (critical point). At pressures and temperatures above the critical point, this single phase has properties which are intermediate between those of the gas and liquid phases and are dependent on the fluid composition, pressure, and temperature. Supercritical fluids are highly compressible just above their critical points. Near the critical point, small changes in pressure result in large changes in density of the fluid. The density of a supercritical fluid is typically about 10.sup.2 to 10.sup.3 times greater than that of the gas. Consequently, molecular interactions increase due to shorter intermolecular distances. However, the diffusion coefficients and viscosity of the fluid, although density dependent, remain more similar to that of gas. Supercritical fluids have greatly enhanced solubilizing capabilities compared to the subcritical gas and higher diffusion coefficients, lower viscosity, and an extended temperature range compared to the corresponding liquid. These properties allow similar solvent strengths as liquids but with greatly improved mass-transfer properties which provide the potential for more rapid extraction rates and more efficient extraction due to better penetration of the matrix.
The substance at this point has basically the properties of a liquid and gas simultaneously. Solvents used at supercritical conditions have very effective solvating properties when exposed to a sample. The low viscosities of the supercritical fluids permit better penetration of the sample matrix for better extraction efficiency. The fast diffusion rates in supercritical fluids allow SFE to take place in minutes as compared to hours in liquids. Often the solvent is many times more effective in extracting a compound from a sample at supercritical conditions than at ambient conditions or even ambient pressure and elevated temperatures. Thus, much smaller samples and amounts of solvents can be used to achieve the same concentration of the extracted compound of interest. Compounds which are difficult or impossible to extract from a sample at ambient conditions, or even in a Soxhlet at elevated temperatures, can be routinely extracted using SFE techniques. SFE is performed in cells which contain the sample and allow exposure of the sample to the solvent at supercritical temperatures and pressures.
Additionally smaller amounts of solvent are used in SFE techniques. Many of the commonly used supercritical fluids are gases at room temperature and pressure, and as a result are much easier to dispose of. The environmental hazards to the public and the laboratory worker is substantially reduced and can be nearly eliminated if careful techniques are followed. Smaller amounts of sample can also be used.
Although SFE techniques have numerous advantages over traditional extraction techniques, such as percolation and Soxhlet techniques, several disadvantages have curtailed its use for routine extraction of multiple samples.
Compounds extracted for analytical purposes are analyzed by a variety of methods, including supercritical fluid chromatography, mass spectrometry, infrared spectroscopy, thin layer chromatography, and many other methods. The extracted compound, or solute, must be introduced directly to the analytical apparatus or collected for further processing or indirect introduction to the analytical apparatus.
Interfacing between the SFE apparatus and the analytical apparatus, or even collection of the solute, has proven to be difficult. The SFE process is carried out at high pressures, often on the order of 10,000 pounds per square inch, and the analytical techniques are most often performed at ambient pressure, or even in a vacuum as in the case of mass spectrometry. At best, interfacing an SFE apparatus for direct introduction into an analytical apparatus is difficult, and in some cases, it is nearly impossible to achieve the interface.
"On-line analysis" is an analytical technique where the solute is introduced directly from the extraction process to the analytical apparatus. On-line analysis using SFE and a supercritical fluid chromatographic apparatus (SFC) has become an effective analytical combination. The supercritical fluid chromatograph can readily accept the SFE solute because of mutual compatibility between SFE and SFC.
The SFE-supercritical fluid chromatographic on-line combination is particularly effective for the analysis of heavy, greater than C35, organic compounds.
On-line analysis of lighter weight compounds of interest can be achieved by introducing the solute at supercritical conditions directly into a gas chromatographic column inside of a gas chromatograph.
The compounds of interest are condensed or deposited on the gas chromatographic column or other trapping means and then separated and eluted from the column and detected using standard gas chromatographic techniques. Gas chromatographic techniques cannot generally analyze heavy, organic compounds because of low volatility.
If the solute eluting from the SFE apparatus is collected for other types of analysis, additional preparation, or use; the solute at supercritical conditions must be brought to ambient pressure and temperatures. Achieving such a reduction in pressure and temperature and effectively collecting the compounds of interest is difficult. This is generally called off-line SFE.
In order to maintain supercritical pressure in the SFE apparatus, the pressure must be reduced slowly at the outlet of the apparatus. The outlet cannot simply be open to the atmosphere. It must be restricted to allow gradual depressurization of the solute. Numerous methods have been used to restrict the pressure drop.
In order to maintain supercritical pressures within the SFE system, any port which allows supercritical fluid to be depressurized and exit the system must act to restrict the exit process. The restriction has to be sufficient to allow the pressure pumps to maintain the supercritical pressure within the SFE system. As the supercritical fluid or solvent passes through the restricted area from the supercritical pressure to a lower pressure, its ability to carry its solutes is reduced and the solutes are deposited at the port. The port can be a small orifice, nozzle, tube, valve, or any other system which allows the fluid to pass through the port in a restricted manner.
The deposition of solutes at the port is one of the major problems encountered in off-line SFE techniques. The solutes are deposited and clog the port. The invention overcomes such problems.
Various techniques have been used to restrict the depressurization of the supercritical solutes to lower pressure where they can be collected or analyzed after off-line SFE.
Wright (Anal. Chem. 59, pp. 38-44, 1987) describes a technique where a stainless steel capillary column is crimped on its exit end in order to form a restriction. This technique has many disadvantages, even though it is commonly used. For example, the stainless steel surface actually forms a catalytic surface which causes decomposition of analytes when heated. Stainless steel tubing is difficult to make and obtain in very small internal diameters. Fifty microns is about the smallest internal diameter stainless steel tubing available. Restriction devices usually require a port with a smaller opening than 50 microns. Thus, the stainless steel tube is crimped at the exit end.
Crimping the tube presents two major problems. First, the crimp cannot be effectively formed in the same manner each time. Therefore, reproducibility of conditions is impossible. Because SFE analytical extractions can use very small volumes, which is advantageous in many cases, small inconsistencies in apparatus conditions can have a large effect on the analysis results. Second, the crimp is required to restrict supercritical fluid within the SFE apparatus as it exits to a lower pressure. The high pressure maintained within the SFE system pushes the crimp open, thus reducing the crimp's effectiveness and varying conditions even within the same extraction.
A changing flow rate during the extraction will make calculation of the total volume of solvent flow difficult or impossible, and conditions of the extraction cannot be reproduced. Samples cannot be compared because they were not obtained under the same extraction conditions. A small change in the opening size of the crimp can make a significant change in the total volume of solvent passing through the system.
Crimping tubes in order to restrict exit of the solute from an SFE system makes meaningful direct quantitative comparison of results from the simultaneous extraction of multiple samples within the same SFE apparatus impossible.
The stainless steel tubing must be heated, as must any other tube acting as a restrictor, to prevent deposition of solute compounds in the tube as the supercritical fluid falls below the supercritical temperature and pressure. Commonly, this heating is achieved by having the tube within the oven, as in the case of on-line analysis procedures using supercritical fluid extraction. The tube is also commonly wrapped with a heat tape of some type which either insulates the tube or actually has a heating capacity and heats the tube to maintain the needed temperature to prevent deposition of solute compounds in the tube. Wright describes a technique of applying an electric current to the stainless steel tube which is crimped. The electric heating technique obviously only works when electrically conductive materials are used to form the restriction tube.
In each case described, heating of the restriction area or tube is not isothermal. It is important that the temperature along the tube, or within the area, be isothermal. If the temperature is variable, compounds being carried in the tube will precipitate where the temperature is below that required to retain such compound or compounds in the dissolved state in the solvent. The diversity of the supercritical fluid actually changes resulting in the precipitation. This deposition clogs the tube and results in erroneous extraction analysis because not all of the samples of interest are eluted from the extraction system.
Very small diameter orifices, usually laser drilled in plates made of metal, sapphire, or other substances, have proven to be ineffective as restrictors. Sputtering occurs when the orifice starts to clog and sample is lost, the plates are hard to attach to the SFE apparatus and cooling is a problem at the exit of the orifice. Clogging is common at the orifice.
Ovens such as gas chromatography or supercritical fluid ovens are specifically designed to maintain a column or tube at an isothermal temperature. In on-line analyses, the restrictor tube can be within such an oven. This is expensive because the oven must be dedicated to use in the SFE process.
Another technique for restriction is use of a fused silica tube. The fused silica tube can be readily obtained with small inside diameters, i.e., 10-50 microns. If the tube has a small enough inside diameter, 10-30 microns, and has a uniform diameter along its length, a linear restriction of the pressure inside the SFE system results along the tube. Pressure is progressively lost, in a direct relationship to the length of the tube, as the solute moves through the tube.
In the on-line system, the SFE system is coupled directly to the chromatographic analysis system. One sample is prepared or extracted and analyzed. Thus, the analytical system is dedicated to the SFE system.
In off-line extraction techniques, numerous extractions can take place simultaneously and several chromatographic systems can be utilized simultaneously. Productivity is greatly improved using an off line system, and the expense of analysis is also greatly decreased.
On-line SFE techniques are reported to be most suitable to gas chromatographic (GC) techniques because the GC techniques analyze the lighter organics or hydrocarbons and with samples of light weight hydrocarbons, the restrictors are not clogged or plugged as easily because light weight hydrocarbons are not as readily precipitated from the solvent. Analysis of heavy weight hydrocarbons obtained from SFE extractors is much more difficult because of the problems with restrictor clogging.
Hawthorne (Anal. Chem., Vol. 60, No. 5, p. 474, Mar. 1, 1988) reports the use of fused silica capillary tubing as a restrictor in an on-line connection of an SFE apparatus to a gas chromatograph (GC). A stainless steel frit was used prior to the restrictor capillary tubing in order to prevent the sample particles from plugging the outlet of the restrictor. Although the capillary tube was effective in controlling the depressurization of the supercritical solute, a new restrictor had to be used for each extraction. Use of a new restrictor with each extraction proves to be time consuming and expensive if multiple samples are analyzed on a routine basis. The fused silica tube is not the expense, as pointed out by Hawthorne (Anal. Chem., Vol. 59, p. 1706, 1987), the tedious labor is the expense. When multiple extractions are being routinely performed, the work of changing the restrictor with each extraction becomes significant.
Not only has the capillary column been found to become fragile and break with a single use, Onuska (Journal of High Resolution Chromatography, Vol 12, p. 357, June 1989) reports that a new restrictor must be used every second extraction, because a single restrictor, if used for several extractions, yields lower recoveries of the compounds of interest due to changes in hydrodynamic profile caused by deposition of material in the restriction tube. The tube is eventually plugged in such cases. In the present invention, the tube does not readily plug and the recovery of the analytes or solutes remains constant, at or near 100%, even after many uses of the same restriction tube.
When analytes are deposited in the restriction tube, successive extraction effluents can become contaminated from deposits made during prior extractions. Thus, a new restriction tube is used in many cases.
Schneiderman, et al. (J. of Chromatography, Vol. 409, pp. 343-353, 1987), have used an off-line SFE process to collect a median weight hydrocarbon for analysis. They have used a valve to restrict the depressurization of the supercritical fluid solute, and the solute or extract was collected on a silica gel trap. The trap was then washed with methylene chloride/acetone (50:50), the solution was evaporated to dryness and the residue was reconstituted in 10 ml of a solvent before analysis. This represents one technique used in state-of-the-art SFE off-line extractions.
Stahl (J. of Chromatography, Vol. 142, pp. 15-21, 1977) has also demonstrated the use of a valve used as a restrictor and has used a thin layer chromatography plate as a trapping mechanism and analyzer. The use of a valve for restriction is a very expensive form of restrictor. A valve does not serve well as a restrictor because it is very difficult to clean. The lubricants used to make the valve function and the residues from past samples or solutes are sources of contamination which are difficult to remove and are very significant in sensitive analytical studies. Additionally, valves are not each made the same and they are difficult to control flow rates with so that the flow rate is reproducible from sample to sample and valve to valve.
Off-line SFE requires some means to trap the compounds or elements of interest that have been dissolved in the supercritical fluid solvent. Schneiderman et al (J. Chrom. Sci., Vol 26, p. 458, Sept. 1988) used a silica gel trap. It is common to use commercially prepared column packing or other adsorbent material such as Tenax.TM. to trap solutes of interest. The extracted compounds or elements of interest are precipitated in the area of packing when the density of the supercritical fluid solvent changes upon exit from the restrictor. The compounds and elements are chemically trapped by the packing, whatever form it takes. The compounds may be lost if there is a "break through" where the exiting solvent fluid or gas (most supercritical fluid solvents are gases, such as CO.sub.2, at ambient temperatures and pressures) passes through the packing carrying the solute and the solute is not exposed to the packing and thus is never trapped on the packing surface. Or, the packing may become saturated, and untrapped solutes can be lost.
Once the analytes or compounds of interest are trapped on the packing surface, they must be removed with some type of a solvent before they can be analyzed or otherwise used. A relatively large amount of solvent must be used and, at least in the case of most analyses, that solvent must be evaporated in order to concentrate the sample and perform a trace analysis. Each step of collection, dissolving, concentration, storage, etc., may result in the loss of some of the compounds of interest, especially light molecular weight hydrocarbons, or contamination of the solute from dirty glassware or solvents. Additionally, each step requires equipment and manpower; thus, making the collection of the compounds of interest more expensive and dangerous.
Use of a packed bed with silica gel, or any other packing, as a trap results in a poor recovery of the compounds of interest. Such poor recovery may be a result of either a failure to trap or secure retention of the compounds of interest on the packing, which does not release the compounds readily when the packing is washed in order to try and bring them back into solutions. Washing the packing, redissolving, and concentrating of the analytes after the SFE process, makes the SFE process much less effective. Its effectiveness is literally diluted and polluted.
Low molecular weight compounds are especially difficult to trap following an SFE process. Two primary techniques have been used to trap such compounds. The depressurizing supercritical fluid solute is bubbled through a solvent or the trapping mechanism is held at a cryogenic temperature. The best recovery of such compounds has been reported at between -30.degree. C. and -60.degree. C. Hawthorne et al (Anal. Chem, Vol. 59, p. 1706, 1987) and Hawthorne and Miller (J. of Chrom. Science, V. 24, p. 258, June, 1986) reports use of the bubbling technique using methylene chloride as the trapping solvent. It is noted that none of the methylene chloride is lost because of the cooling effect caused by the rapid expansion of the supercritical fluid as it exists in the restrictor.
The rapid depressurization of the supercritical fluid at the end of the restrictor has an adiabatic cooling effect which compounds the plugging or clogging of the restrictor. Even though the end of the restrictor may be in a solvent, the cooling effect is so pronounced that plugging is still a problem. Plugging is not as big a problem when only light molecular weight compounds are in the sample, even though a lower flow rate is commonly used to facilitate collection of such compounds. Lower flow rates will almost guarantee plugging of the fused silica or crimped stainless steel tubing restrictors if heavy molecular weight compounds are present in the sample.